Conventional methods for analyzing biological samples such as blood or body fluids typically include two steps. First, an automated system performs a quantitative analysis of sample characteristics on the sample in a liquid state. For example, a flow cytometer using impedance, fluorescence, and/or scattered light-based measurements processes a sample of blood suspended in a fluid steam to count red blood cells, white blood cells, platelets, and to derive other parameters of a complete blood count. Second, to the extent the flow system detects any abnormalities in the sample (e.g., an abnormally high white blood cell count), the system flags the sample and a laboratory technician reviews the sample manually by examining a dried and stained preparation of the sample on a microscope slide.
To facilitate manual review, the technician typically prepares a “wedge” smear of the blood sample. Smear preparations yield samples with highly variable thicknesses and distribution of blood constituents. The wedge smear often has only a single narrow band with an appropriate cell density for examination and analysis, and the location and shape of this band varies from slide to slide. In addition, due to a lack of uniformity, smear preparations often preclude absolute quantitation of sample properties for a given patient. In general, only relative proportions can be assessed within the smear itself.
A sample preparation method that produces a uniform, high-quality specimen would make visual evaluation of the sample both easier and more accurate. Furthermore, for specimens prepared using a known volume of the sample in a highly consistent manner, it is possible to automate the quantitation of sample properties directly from the specimen, replacing the first step of sample analysis traditionally performed using flow-based systems.